The cathodic protection of metal sections of structures is well known. This technique provides corrosion protection for the metal section by the formation of an electrical circuit that results in the metal section acting as a cathode and therefore oxidation of the metal does not occur.
One such known type of system for cathodic protection is the impressed current system, which makes use of an external power supply, either mains or battery, to apply current to the metal section to be protected so as to make it cathodic. These systems generally require complex circuits to apply the current appropriately and control systems to control the application of the current. Furthermore, those that are supplied with mains power clearly can encounter difficulties with power supply problems such as power surges and power cuts, whilst those powered by battery have to overcome the issue of locating the battery at an appropriate position, which both allows the battery to function correctly and supports the weight of the battery.
Often, therefore, such impressed current systems have a battery secured to the exterior of the structure containing the metal sections to be protected, which clearly adversely affects the look of the structure.
Other systems for cathodic protection, which avoid the need for bulky or complex components make use of a sacrificial anode coupled to the metal section. The sacrificial anode is a more reactive metal than the metal of the metal section and therefore it corrodes in preference to the metal section, and thus the metal section remains intact.
This technique is commonly used in the protection of the steel reinforcements in concrete, by electrically connecting the steel to a sacrificial anode, with the circuit being completed by electrolyte in the pores of the concrete. Protection of the steel reinforcements is in particular required when chloride ions are present at significant concentrations in the concrete, and therefore cathodic protection is widely used in relation to concrete structures in locations which are exposed to salt from road de-icing or from marine environments.
A problem associated with such cathodic protection arises from the fact that it is the voltage between the sacrificial anode and the metal section that drives current through the electrolyte between these components. This voltage is limited by the natural potential difference that exists between the metal section and the sacrificial anode. Accordingly, the higher the resistance of the electrolyte, the lower the current flow is across the electrolyte between a given metal section and sacrificial anode, and hence the application of sacrificial cathodic protection is restricted.
Accordingly, there is a need for a sacrificial anode assembly that can give rise to a voltage between itself and the metal section greater than the natural potential difference that exists between the metal section and the material of the sacrificial anode.